SML Syndrome is an autosomal recessive disorder which causes congenital abnormalities, severe failure to thrive, and significant mental retardation. We have recently described a defect in cholesterol biosynthesis in patients with SLOS at the penultimate step in cholesterol production, causing markedly reduced cholesterol levels and accumulation of abnormal cholesterol precursors, particularly 7-dehydrocholesterol (7-DHC). This is a newly described metabolic error in humans.